Secondary emission electron beam monitor



Jan. 12, 1965 SIGERU OKABE 3,165,629

SECONDARY EMISSION ELECTRON BEAM MONITOR Filed Jan. 8, 1962 INVENTOR.

United States Patent 3,165,629 SECONDARY EMISSION ELEtJ'IRON BEAM MGNITOR Sigern Olra'be, Sakaishi, Osakafn, Japan, assignor to Osaka'iuchiji Gisen Sate, Osakaru, Japan, a corporation of Japan Filed Jan. 8, 1962, SerrNo. 164,923 Claims priority, application Japan, Dec. 7 1961, 36/ 44,497 4 Claims. (61. 250-495) This invention relates to a monitoring device for monitoring an electron beam of relatively low energy which is used for irradiation, characterized in that it makes use of the phenomenon of emission of secondary electrons, and that it uses the window for letting the electron beam out of the accelerator into the outside air as an electrode for emitting or collecting secondary electrons and provides another electrode on the vacuum side of the window.

It is an object of the present invention to provide a device to enable a measurement as precise as possible during irradiation of high energy electron beams of about 1-3 mev. which are becoming used more frequently in industry. Another object is to enable the measurement in such a manner that there is little disturbance of the electron beam during measurement. A further object is to make it possible to measure the electron beam within an observational error less than 12%. A still further object is to provide a simple apparatus for measuring the electron beam with greater facility. Yet another object is to provide means for the measurement which is non-saturable even when electron beams of high current densities more than 20 ma./cm. may be used.

A preferred embodiment of the invention will be better understood by reference to the attached drawings, in which: FIG. 1 is a cross-sectional view of the monitor. FIG. 2 is a schematic diagram of the monitoring system, and FIG. 3 is a diagram showing performance characteristics of the monitoring device.

Recently there has been a tendency to utilize irradiation processes utilizing high current electron beams in various industrial fields. However, conventional accelerators used for the above purpose are not equipped with precise and efficient monitors for high current electron beams during irradiation. The electron beams were usually monitored only by indirect procedures or by ones which directly operate on the beams but cause some dis turbances to them. This method has many disadvantages such as unreliability, non-practicality and unfitness for the measurement of high electron beam current. Thus, there has long been needed a simple and relatively inexpensive measuring device with more reliability, more facility and less observational error. The present invention is for meeting this demand.

Now, in describing the invention in detail, reference will be had to the drawings in which arrow 1 indicates the inlet direction of the electron beam accelerated by the accelerator. A brass tube 2 with a diameter the same as or a little larger than that of the electron conduit of the accelerator has a flange 3 on one end threoi adapted to be coupled directly to the accelerator. A sealing diaphragm 4 consisting of an aluminum foil generally of 29 rug/cm. in weight is positioned at the other end of tube 2, through which the electron beam passes from the vacuum in the accelerator into the atmospheric pressure of 1 atm. In this embodiment of the invention this diaphragm 4 is used as an emitter of secondary electrons. A collector electrode in the form of a thin sheet 5 of aluminum foil (1.4 mg./cm. which may also be a grid or other suitable electrode is mounted closely adjacent, at a distance of about 35 mm. to the window foil. This thin foil is held tightly by a ring-shaped polystyrene insulator 6 and a brass ring 7 and is within the vacuum chamber in the tube 2. In the present invention, this foil is used as a collector for the secondary electrons by applying thereto a positive potential of v. or so, as shown in FIG. 2. A hermetic seal 9 is provided through which extends an electrical connection to the exterior of the chamber. A gum packing or O-ring 1t) seals the diaphragm 4. 7

During the operation of the apparatus low energy secondary electrons are emitted from the diaphragm foil 4 which is positioned in the path of the electron beam used for irradiation. The electrons are attracted to the thin foil 5, and a fixed portion of them will be collected on it.

In the diagram of FIG. 3, the abscissa denotes primary beam current I measured in a. (microamperes) and the ordinate the secondary electron current I in ,ua. The solid dots represent the current measured at a beam energy of 1.0 mev., and the circles the current at a beam energy of 1.5 mev., respectively. It will be seen from the diagram that the secondary electron current (1,) varies in proportion to the primary beam current (1 and so when the collected current (1,) is measured by a microammeter or current integrator 11 in FIG. 2, then the beam current (I can automatically be determined.

Further, it must be noted that the disturbance to the primary beam caused by inserting the monitor in the beam passage is negligible both on the spreading out of or on the absorption, due to the thinness of the collector electrode aluminum foil (or grid). Furthermore, this invention has the advantage that it is non-saturable at high current densities, such as current densities more than 20 ,ua./cm. and the monitor output is independent of the primary beam energy and proportional to the primary beam current.

There has thus been provided a simple yet accurate monitor which has one of the electrodes used in monitoring. also acts as the diaphragm between the vacuum space within the particle accelerator and the outside atmosphere. This reduces the amount of material through which the beam must pass and therefore keeps the disturbance of the beam to a minimum.

It is thought that the invention and its advantages will be understood from the foregoing description and it is apparent that various changes may be made in the form, construction and arrangement of the parts without departing from the spirit and scope of the invention or sacrificing its material advantages, the form hereinbefore described and illustrated in the drawings being merely a preferred embodiment thereof.

I claim:

1. A monitoring device for measuring the current intensity of an electron beam having an energy of from 1-3 mev. and generated in an electron accelerator with an electron conduit, consisting essentially of a tube having a diameter substantially the same as the conduit of the electron accelerator, means on one end of the tube adapted to attach said monitoring device to the electron accelerator, an aluminum foil diaphragm having a Weight of about 29 mg./cm. sealed over the other end of said tube for enabling a vacuum to be maintained in said tube, said diaphragm acting as one electrode, a second aluminum foil electrode mounted across the tube and spaced from said first electrode from 3-5 cm. toward the said one end of said tube, said foil having a weight of about 1.4 mg./cm. and a voltage bias means having a Voltage of about 150 v. and a current measuring means connected to said second electrode, said first electrode being grounded.

2. A monitoring device for measuring the current intensity of an electron beam having an energy of from about 1-3 ,uV. and generated by an electron accelerator with an electron conduit, consisting essentially of a tube having a diameter substantially the same as the conduit of Patented Jan. 12, 1965 3 4 the electron accelerator, me'anson one end of the tube, to said second electrode, said first electrode being adapted to'attach said monitoring device to the electron grounded. accelerator, a metal foil diaphragm over the other end of 3. A monitoring device as claimed in claim 2 in which the tube having a thickness of about 29 mg./cm. for said second electrode is afoil. enabling a vacuum to be maintainedin said tube; said 5 4. A monitoring device as claimed in claim 2 in which diaphragm acting as one electrode, a second electrode said second electrode isagrid. of thin sheet-like metal mounted across the tube closely adjacent the first electrode and spaced from said first Refefeflcfls Cited in the file of this'patent electrode toward said one end of said tube, said metal UNITED STATES PA of said second electrode having a Weight of about 1.4 10 mg./crn. and a voltage bias means having a voltage of 2 a 3 about 150 v. and a current measuring means connected a e 

1. A MONITORING DEVICE FOR MEASURING THE CURRENT INTENSITY OF AN ELECTRON BEAM HAVING AN ENERGY OF FROM 1-3 MEV. AND GENERATED IN AN ELECTRON ACCELERATOR WITH AN ELECTRON CONDUIT, CONSISTING ESSENTIALLY OF A TUBE HAVING A DIAMETER SUBSTANTIALLY THE SAME AS THE CONDUIT OF THE ELECTRON ACCELERATOR, MEANS ON ONE END OF THE TUBE ADAPTED TO ATTACH SAID MONITORING DEVICE TO THE ELECTRON ACCELERATOR, AN ALLUMINUM FOIL DIAPHRAGM HAVING A WEIGHT OF ABOUT 29 MG./CM.2 SEALED OVER THE OTHER END OF SAID TUBE FOR ENABLING A VACUUM TO BE MAINTAINED IN SAID TUBE, SAID DIAPHRAGM ACTING AS ONE ELECTRODE, A SECOND ALUMINUM FOIL ELECTRODE MOUNTED ACROSS THE TUBE AND SPACED FROM SAID FIRST ELECTRODE FROM 3-5 CM. TOWARD THE SAID ONE END OF SAID TUBE, SAID FOIL HAVING A WEIGHT OF ABOUT 1.4 MG./CM.2, AND A VOLTAGE BIAS MEANS HAVING A VOLTAGE OF ABOUT 150 V. AND A CURRENT MEASURING MEANS CONNECTED TO SAID SECOND ELECTRODE, SAID FIRST ELECTRODE BEING GROUNDED. 